Grease guns have typically been primarily constructed of expensive, relatively heavy metal parts such as a metal barrel into which a grease cartridge could be placed, a metal grease pressurizing compression spring and piston, a metal spring compressing (cocking) rod, a metal grease pump head and metal pump handles. Typically, the metal pump head incorporates a piston-type pumping mechanism, a single check valve, and an air bleed port which may aid in evacuating the trapped air introduced during the loading process. The single check valve configuration does not allow for the expelling of the trapped air without the use of the air bleed port. The cartridge loading operation for such structures, particularly where the pressurizing spring compressing (cocking) rod must be retracted and held in place while the cartridge is positioned in the barrel, can be a difficult and messy operation for many users. Also, such structures do not allow the operator to view the contents of the barrel, or the legends on the cartridge wall which denote, e.g., manufacturer, brand, type, lot number, and expiration date, the amount of grease (consumed and remaining) in the cartridge, and the condition of the grease, e.g., whether separation (lubricant from thickener) has occurred in the cartridge. Further the present invention, in a preferred embodiment, may be constructed so as to prevent removal of a cartridge and its replacement with a cartridge having the wrong and deleterious type of grease for some highly sensitive applications.
Typical of such prior art grease gun structures are shown in U.S. Pat. Nos. 1,697,217; 1,714,214; 4,601,412; 3,187,959; 1,633,356; 1,812,643; 3,393,840; 3,341,084; 2,284,533, the disclosures of which are hereby incorporated herein by reference in their entireties.